With reference to FIGS. 16 and 17 showing an electric toothbrush for practicing the Bath method (reciprocating motion), the toothbrush comprises a tubular case 1 serving also as a handle, a motor 2 housed in the case 1 and having a motor shaft 21 extending along the axis of the case 1, and a reciprocating shaft 5 having a brush member 7 and reciprocatingly drivable by the rotation of the motor shaft 21 as converted into a linear movement approximately along the axis of the case 1 by motion conversion means.
For example according to JP-A No. 9-24057 and JP-A No. 9-177923, a pinion gear 31 mounted on the motor shaft 21 is in mesh with a crown gear 32 perpendicular to the axis of the gear 31. A rotating shaft 33 for the crown gear 32 has mounted thereon an eccentric rotating body 34, which is rotatably fitted in a cavity 35 in a base end of the reciprocating shaft 5 reciprocatingly movable approximately along the axis of the case 1 to provide a scotch yoke mechanism, whereby the rotation of the motor shaft 21 is converted into linear motion.
With electric toothbrushes of the Bass type, the frequency of reciprocating motion of the brush member is considered important.
With the conventional motion conversion means, the rotation of the motor shaft 21 is delivered to the scotch yoke mechanism (cross slider mechanism) upon a reduction at the gear ratio between the pinion gear 31 and the crown gear 31, so that the frequency of the reciprocating motion of the brush member 7 invariably becomes smaller than the number of revolutions of the motor shaft 21.
Because of the construction wherein the scotch yoke mechanism is provided coaxially with the rotating shaft 33 of the crown gear 32, the crown gear is positioned out of alignment with the axis of the tubular case 1, such that as the crown gear 32 is positioned a larger distance away from the case axis, the crown gear 32 needs to have a smaller outside diameter. The larger the crown gear 32 in outside diameter, the more advantageously the gear 31 meshes with the pinion gear 31, whereas this invariably reduces the number of teeth because of the limitation imposed by the outside diameter. Further to give a satisfactory outside diameter to the pinion gear 31 to be in mesh with the crown gear 32, the crown gear 32 needs to be positioned a greater distance away from the axis of the tubular case 1. Consequently, the pinion gear 31 and the crown gear 32 become smaller in the number of teeth, and the meshing engagement between the gears 31, 32 involves a greater backlash, producing vibration and noise and impairing the smoothness of reciprocating motion.
In the case where the scotch yoke mechanism is shaped to have the smallest possible thickness in order to avoid the above problems, the mechanism becomes liable to brittle fracture due to a reduction in the strength of the material. Furthermore a diminished tooth profile module will result in view of the need to assure the pinion gear 31 of strength. This increases the gear ratio between the pinion gear 31 and the crown gear 32 to reduce the frequency of the reciprocating motion.
Further because the tubular case 1 serves also as a handle, the thickness of the case 1 is limited from the viewpoint of ease of gripping and design, consequently limiting the gear ratio between the pinion gear 31 and the crown gear 32 to a certain range. For this reason, the conventional construction involves diminished freedom of design in giving an increased frequency to the brushing motion, consequently posing an obstacle in improving the performance with respect to vibration and noise.